Sideros is the God of Hope and is incarnated in the Stars, he represents the mercy of the light and the infinite hope for a better future. He is followed mostly by priests and healers, whose powers are enchanched by his gifts of heal. Sideros is the less fightful and inclined to combat between the pantheon of Light.
Who seeks peace of the spirit meditating and praying to Sideros will feel most peaceful whitin, also Sideros can restore a mortal's mind even if is completely broken, lifting despair and madness away.
The high Elves are under Sideros' grace, and after the propeth Allistar, guiding the Dawnstrider Order, managed to rally the High Elves under the religion of Sideros, their magic addiction has been slowly reduced, to the point that now High Elves do not feel their hunger anymore, so long they follow the light of Sideros in their belief.
Sideros choose the High Elves because they are one of the most doomed races on azeroth, their population being low and having no homeland at all, still, they bring hope for the future.